Identification of corticosteroid-binding globulin in human milk: measurement with a filter disk assay

We have studied the concentration and properties of a protein which binds cortisol in human milk in samples obtained from women during the first 100 days after delivery. A filter disk assay was developed both for the measurement of plasma corticosteroid-binding globulin (CBG) and for the cortisol-binding protein in milk. The concentration of CBG in milk, expressed as its capacity to bind cortisol, is highest on the day of delivery, ca, 0.80 mug/dl, falls over the next 10 days to ca. 0.25 mug/dl, and remains at that level thereafter. If the concentration of CBG is expressed relative to the concentration of serum albumin in milk, it increases from day 1 to day 3 and then remains constant. A detailed comparison of CBG derived from milk and plasma showed that the two proteins co-migrated on Sephadex, sucrose gradient ultracentrifugation, and polyacrylamide gel electrophoresis. The two proteins had the same affinity for cotrisol, progesterone, 17-OH-progesterone, and dexamethasone. Furthermore, the binding activity of CBG in milk was neutralized with anti-CBG antibodies raised against CBG isolated from plasma. Unlike CBG, the concentration of cortisol in milk, 0.8-3.5 mug/dl, showed no systematic variation as a function of the postpartum day on which the sample was obtained.